The present invention relates to a ferromagnetic resonance device suitable for use with a microwave filter or a microwave oscillator, and particularly to a ferrimagnetic resonance device utilizing ferrimagnetic resonance of YIG (yttrium iron garnet) thin film.
Conventionally, a magnetic resonance element for a microwave device such as a filter or an oscillator utilizing ferrimagnetic resonance of YIG employs a spherical body prepared from a bulk single crystal of YIG. However, a lower limit of resonance frequency of the spherical body is relatively high owing to the demagnetizing field, and for instance, it is 1680 MHz in case of using an unsubstituted YIG sphere having a saturation magnetization of 1800 G (gauss). Thus, the prior art has not yet attained a microwave device capable of operating in a range down to a UHF band. On the other hand, the lower limit of resonance frequency may be reduced by partially substituting a non-magnetic ion such as Ga.sup.3+ for Fe.sup.3+ in YIG and thereby decreasing the saturation magnetization. In this case, if an amount of substitution is too large, a half width .DELTA.H of resonance is increased to cause deterioration in characteristics of the device.
In another technique, it has been proposed that a microwave device utilizing ferrimagnetic resonance is provided by forming a YIG thin film on a GGG (gadolinium gallium garnet) substrate by liquid phase epitaxial growth (which will be hereinafter referred to as LPE) and working the thin film into a desired pattern such as a circular or rectangular shape by photolithography. As such a microwave device may be prepared as a microwave integrated circuit (which will be hereinafter referred to as MIC) using a micro-strip line or the like for a transmission line, the device is easily mounted in a magnetic circuit for applying a D.C. bias magnetic field. Further, as the device is produced by using LPE and photolithography, mass-produceability is improved. Such ferromagnetic resonance device utilizing YIG thin film is shown in U.S. Pat. Nos. 4,547,754, 4,626,800, 4,636,756, U.S. Ser. Nos. 708,851 filed Mar. 6, 1985, 740,899 filed June 3, 1985, 844,984 filed Mar. 27, 1986, 883,605 filed July 9, 1986 and 833,603 filed July 9, 1986, all assigned to the assignee of the present application. Additionally, the use of the thin-film element can greatly reduce the lower limit of resonance frequency as compared with a spherical element. However, in such a magnetic resonance device using the YIG thin film element, a detailed investigation intended to reduce the lower limit of resonance frequency to an ultimate value has not yet been reported.
As is mentioned above while the investigation for reducing the lower limit of resonance frequency to an ultimate value has not yet been established, an exemplary method of reducing the lower limit to an ultimately low frequency is to strengthen connection between the YIG thin film element and the transmission line and thereby sufficiently decrease an external Q value of a resonator. That is, since an unloaded Q value of a YIG resonator is lowered in a low frequency, it is necessary to sufficiently reduce the external Q value, so as to enlarge to some extent a reflection amplitude in case of a reflection type or a transmission amplitude in case of a transmission type.
FIG. 10 shows a structure of a YIG thin film resonance device of a YIG thin film type band-pass filter. In this structure, a ground conductor 2 is formed on one of principal planes (which will be hereinafter referred to as a first principal plane) of a dielectric substrate 1 such as an alumina substrate, and first and second parallel micro-strip lines 3 and 4 acting as input and output transmission lines, respectively, are formed on the other principal plane (which will be hereinafter referred to as a second principal plane). The micro-strip lines 3 and 4 are connected at their ends through first and second connecting conductors 5 and 6, respectively, to the ground conductor 2. First and second YIG thin film elements 7 and 8 as a magnetic resonance element are arranged on the second principal plane of the substrate 1, and are electromagnetically connected to the first and second micro-strip lines 3 and 4, respectively. These YIG thin film elements 7 and 8 are prepared by forming a YIG thin film on one of principal planes of a non-magnetic GGG substrate 9 by the afore-mentioned thin film forming technique and making a desired pattern such as a circular shape of the thin film by a selective etching using photolithography technique, for example. A third micro-strip line 10 as a connecting transmission line for electromagnetically connecting the first and second YIG thin film elements 7 and 8 as the first and second magnetic resonance elements with each other is formed on the other principal plane of the GGG substrate 9. The third micro-strip line 10 is connected at its both ends through third and fourth connecting conductors 11 and 12, respectively, to the ground conductor 2. The shown structure in FIG. 10 is placed in a D.C. bias magnetic field applied perpendicular to a major surface of the YIG thin film element, though the bias magnetic field applying structure is not shown in FIG. 10.
However, as the connection between the microstrip lines and the YIG thin film elements is not so strong, the external Q value cannot be reduced to such an extent as to be required by a low-frequency operation. In the case of the YIG thin film elements 7 and 8 having a diameter of 2.5 mm and a thickness of 25 .mu.m, an external Q value Q.sub.e1 due to the connection between the YIG thin film elements 7 and 8 and the input and output transmission lines 3 and 4 was 200, while an external Q value Q.sub.e2 due to the connection between the YIG thin film elements 7 and 8 and the connecting transmission line 10 was 250. In order to further reduce these external Q values, it is necessary to enlarge the volume of the YIG thin film elements 7 and 8. However, if the diameter of the elements 7 and 8 were made so large in comparison with a width of the microstrip lines as the transmission lines, a spurious characteristic would be deteriorated. Further, if the thickness of the elements 7 and 8 were increased, a resonance frequency would be disadvantageously increased.